The invention relates to the production of short pulses by compression of laser pulses of relatively long duration. It more particularly relates to the production of mono-pulses of light (that term being construed as indicating IR and UV light as well as visible light) having a short duration. It has numerous applications, particularly in all fields requiring generators or amplifiers of brief and intense light pulses.
Methods have been used to obtain brief laser pulses with a good energy conversion efficiency. Most are based on the same principle, namely use of a lasing medium having considerable gain and a laser resonant cavity having a low Q. The rising time duration of the laser pulse is directly related with the time necessary to saturate the gain and the decay time reflects the life duration of the photons in the resonant cavity. Beside locked-mode lasers, which emit pulse trains whose unit duration is in the picosecond range of magnitude, the restrictions imposed by the pumping time, the reaching of gain saturation and the life duration in the cavity have the result that very few lasers exist capable of emitting single pulses whose duration does not exceed ten nanoseconds. It is obviously possible to shorten by cutting up the duration of the pulses supplied by the laser. Cutting up by electro-optical elements (Pockels cell, Kerr cell) permits the obtaining of pulses of some nanoseconds but causes a considerable energy loss since it does not provide compression. It has also been proposed to use Raman back-scattering to obtain time compression but corresponding devices are bulky and, taking into account the characteristics of the lasers used, the results obtained are generally limited (U.S. Pat. No. 4,306,195 to Stappaerts).
Stimulated Raman emission has also been suggested for achieving substantial average output powers at kHz pulse repetition frequencies with pulse lengths which may be in the nanosecond range. In stimulated Raman emission or scattering, a "pump" beam at a frequency .nu.p passing through a Raman medium results in a gain at a Stokes frequency .nu.s which is proportional to the pump beam intensity. Frequency .nu.s is selected for .nu.p-.nu.s to be equal to the frequency of a Raman transition.
As long as the number ns of photons generated at frequency .nu.s is low as compared with the number np in the pump beam, the depopulation of the pump beam is negligible and the value of the Stokes pulse is an exponential funcction of the result of the product (pump intensity Ip).times.(interaction length). Then the efficiency of the transformation process, as represented by ns/np (ns being the number of photons generated at frequency .nu.s) is low. That approach is described in a paper entitled "0.9 W Raman Oscillator" by E. O. Ammam et al in Journal of Applied Physics, Vol. 48, No. 5, May 1977, p. 1973. It appears from the upper curve in FIG. 2 that the greater part of the pump energy remains stored in the cavity, depopulation being about 15%, and the quantum efficiency ns/np is low.
It is an object of the invention to generate short light pulses by time compression of laser energy by at least one order of magnitude with a high degree of efficiency. It is a more particular object to provide a process and apparatus delivering light pulses whose time length does not exceed some ns from pulses whose duration is greater by at least one order of magnitude with a quantum efficiency higher than 85%.
For this purpose, a method for producing brief mono-pulses of light includes the steps of locating, in a laser cavity, a Raman medium associated with a cavity dimensioned so as to present a considerable gain at the Stokes frequency and extracting most stored laser energy at the Stokes frequency in a single to and fro travel through the cavity.
The time compression obtained enables the light intensity of the pulse to be notably increased.
The invention may be put into practice in an amplifying device as well as a light generator or again in a complete system comprising an oscillator and one or more amplifiers.
The invention also provides a device for producing mono-pulses of light of short duration, comprising a laser cavity; in the laser cavity, there is placed a Raman medium also inserted in a Stokes cavity having a low Q, capable of converting energy at the laser frequency into energy at the Stokes frequency. This permits in particular the production of novel infrared sources. The laser cavity has preferably the highest possible Q.
In the case of operation as an amplifier, it is possible to amplify a spectrally pure Stokes wave by equipping the device with spectral compression means of the type described in U.S. Pat. No. 4,348,599.
The means for extracting the pulse at the Stokes frequency can be constituted by a semi-transparent mirror at this frequency, constituting one of the walls defining the Stokes cavity, having a reflection factor not exceeding some percent for the Raman frequency in order that the Q may be low. The zone common to the laser and Stokes cavities can be bounded by separators which are transparent for the laser or Raman frequency, reflective for the other.
Several Raman cavities can be overlapped so as to obtain energy transfers in cascade: such an assembly enables successive frequency shifts towards the infrared to be achieved. In addition, it is possible to use frequency unable laser media placed in dispersive cavities or injected by an external source and hence to create tunable radiation pulses, always of shorter duration than the initial laser pulse. With a coupled-mode laser, adaptation of the cavity lengths makes it possible to achieve durations as short as a picosecond, all the energy being recovered in a single pulse. Finally, the invention can be used to improve the performances of low gain laser (Alexandrite for example) and those of short life-span lasers (excimer lasers for example).
The Raman medium may be placed in the laser cavity. However, that restriction --which is a drawback when the laser medium is absorbent at laser frequency, as is for example the case with excimer lasers-- may be overcome if it is borne in mind that the process may be considered as having two phases, namely:
storing laser energy in the form of radiation, which storage is done by accumulation in the low loss laser cavity, whilst the gain of the laser is higher than the losses of the cavity;
extracting the laser energy at a frequency shifted by stimulated Raman scattering, that is to say by a non-linear optical process, which effects the frequency conversion with an amplification coefficient which increases exponentially with the stored energy.
In a modified embodiment of the invention, the second phase of the method (energy extraction from the cavity at the Stokes frequency in a time corresponding to a single to and fro travel through the cavity, in a high gain medium) remains unchanged. However, the first phase is modified: a non-linear medium capable of effecting a frequency change is located in the storage cavity and the beam of the laser (which is outside the storage cavity) is injected into the non-linear medium whereby there is an intermediate step.
The storage cavity will be bounded by means which give it a high Q at the storage frequency. A convenient solution consists of injecting the laser beam into the storage cavity through a dichroic mirror transparent at the laser frequency, but almost totally reflecting at the storage frequency.
The non-linear medium for conversion of the laser frequency, which constitutes a pump frequency, to the storage frequency, may utilize very various processes, for example:
stimulated scattering by the Raman effect,
a parametric process, which reduces the frequency to a value equal to a passive complementary frequency,
frequency doubling, which requires starting from a sufficient low pumping frequency so that media transparent at the storage frequency may be available,
frequency tripling, which requires starting from a laser frequency in the infrared or visible range, for the same purpose as that described with regard to frequency doubling.
A modified device for producing mono-pulses of light of short duration, comprising high Q cavity for the storage of light energy provided by a laser, in which cavity is placed a Raman medium also inserted in a Stokes cavity with a low Q for converting energy at the storage frequency into energy at the Stokes frequency; the laser is placed outside the storage cavity and the latter contains a non-linear medium for receiving the laser beam, adapted to convert the laser energy into energy at a storage frequency for which the storage cavity has a high Q.
The invention will be better understood on reading the description which follows of preferred embodiments given by way of examples.